The increase in computing power, spatial densities in semiconductor-based devices and energy efficiency of the same allow for ever more efficient and small microelectronic sensors, processors and other machines. These have found wide use in mobile and wireless applications and other industrial, military, medical and consumer products.
Even though computing energy efficiency is improving over time, the total amount of energy used by computers of all types is on the rise. Hence, there is a need for even greater energy efficiency. Most efforts to improve the energy efficiency of microelectronic devices has been at the chip and transistor level, including with respect to transistor gate width. However, these methods are limited and other approaches are necessary to increase device density and processing power and to reduce power consumption and heat generation.
One field that can benefit from the above improvements is in switched inductor power conversion devices. These devices can be challenging because power loss increases with higher currents, pursuant to Ohm's law: Ploss=I2R, where Ploss is the power loss over the length of wire and circuit trace, I is the current and R is the inherent resistance over the length of wire and circuit trace. As such, and to increase overall performance, there has been a recognized need in the art for large scale integration of compact and dense electrical components at the chip level, such as for use with the fabrication of complementary metal oxide semiconductors (CMOS).
With the development of highly integrated electronic systems that consume large amounts of electricity in very small areas, the need arises for new technologies which enable improved energy efficiency and power management for future integrated systems. Integrated power conversion is a promising potential solution as power can be delivered to integrated circuits at higher voltage levels and lower current levels. That is, integrated power conversion allows for step-down voltage converters to be disposed in close proximity to transistor elements.
Unfortunately, practical integrated inductors that are capable of efficiently carrying large current levels for switched-inductor power conversion are not available. Specifically, inductors that are characterized by high inductance (e.g., greater than 1 nH), low resistance (e.g., less than 1 ohm), high maximum current rating (e.g., greater than 100 mA), and high frequency response whereby there is little or no inductance decrease for alternating current (AC) input signal up to 10 MHz are unavailable or impractical using present technologies.
Furthermore, all of these properties must be economically achieved in a small area, typically less than 1 mm2, a form required for CMOS integration either monolithically or by 3D or 2.5D chip stacking. Thus, an inductor with the aforementioned properties is necessary in order to implement integrated power conversion with high energy efficiency and low inductor current ripple which engenders periodic noise in the output voltage of the converter, termed output voltage ripple.
Accordingly, there is a need for high quality inductors to be used in large scale CMOS integration, to provide a platform for the advancement of systems comprising highly granular dynamic voltage and frequency scaling as well as augmented energy efficiency.
The use of high permeability, low coercivity material is typically required to achieve the desired inductor properties on a small scale. In electromagnetism, permeability is the measure of the ability of a material to support the formation of a magnetic field within itself. In other words, it is the degree of magnetization that a material obtains in response to an applied magnetic field. A high permeability denotes a material achieving a high level of magnetization for a small applied magnetic field.
Coercivity, also called the coercive field or force, is a measure of a ferromagnetic or ferroelectric material's ability to withstand an external magnetic or electric field, respectively. Coercivity is the measure of hysteresis observed in the relationship between applied magnetic field and magnetization. The coercivity is defined as the applied magnetic field strength necessary to reduce the magnetization to zero after the magnetization of the sample has reached saturation. Thus coercivity measures the resistance of a ferromagnetic material to becoming demagnetized. Ferromagnetic materials with high coercivity are called magnetically hard materials, and are used to make permanent magnets. Ferromagnetic materials that exhibit a high permeability and low coercivity are called magnetically soft materials, and are often used to enhance the inductance of inductors.
Coercivity is determined by measuring the width of the hysteresis loop observed in the relationship between applied magnetic field and magnetization. Hysteresis is the dependence of a system not only on its current environment but also on its past environment. This dependence arises because the system can be in more than one internal state. When an external magnetic field is applied to a ferromagnet such as iron, the atomic dipoles align themselves with it. Even when the field is removed, part of the alignment will be retained: the material has become magnetized. Once magnetized, the magnet will stay magnetized indefinitely. To demagnetize it requires heat or a magnetic field in the opposite direction.
High quality inductors are typically made from high permeability, low coercivity material. However, high permeability materials tend to saturate when biased by a large direct current (DC) magnetic field. Magnetic saturation can have adverse effects as it results in reduced permeability and consequently reduced inductance.